Problem: Solve for $z$ : $z + 15 = 24$
Solution: Subtract $15$ from both sides: $ \begin{eqnarray} \\ z + 15 &=& 24 \\ \\ {-15} && {-15} \end{eqnarray} $ $ \begin{eqnarray} \hphantom{15 + z} &\hphantom{=}& \hphantom{24} \\ z &=& 24 {- 15} \\ \end{eqnarray} $ Simplify. $ z = 9$